The present invention is directed to a method of controlling resistivity in associated methods of consolidating particulate metals, ceramic materials, or combinations of such materials into shaped products of very low porosity. More specifically, this invention provides controlled resistivity in connection with a means to apply high compaction pressures at temperatures in the range of sintering temperature of the materials being consolidated to achieve essentially complete densification at extremely rapid processing rates as compared with prior art technologies. Moreover, because this invention employs a method of electrothermal heating generally referred to as "charge-resistor" heating, the temperatures that can be obtained by the process of this invention are well above the requirements for even the most refractory and heretofore difficult materials to densify a wide variety of metallic and ceramic materials, such as silicon carbide, boron carbide and other very high melting point materials.
Processes for making dense metallic and/or ceramic articles from their powders are well established in industrial practice of the prior art. It is possible, for example, by the method known as "hot pressing," to place the powder between two cylindrical pistons contained within a cylindrical chamber having only a slightly greater diameter than the pistons, and by application of heat and pressure directly to the powder, the powder can be formed into a highly dense cylindrical billet. Because the method applies an external pressure directly to the material at or near its sintering temperature, hot pressing will yield consolidated product of higher density than if the powder material is densified by pressureless sintering. Hot pressing is a relatively simple prior art method to achieve essentially complete densification and is widely used to make many articles in present commercial use. However, because high temperatures are needed to consolidate high melting point metal alloy powders and the recently developed high performance ceramic materials, the dies and plungers used for hot pressing must be made of a material capable of withstanding the required temperature. Graphite is the most practical material and is commonly used in commercial hot pressing of ceramics. To avoid oxidation of graphite tooling, the hot processing apparatus must be enclosed within a controlled atmosphere chamber, and shielded therein by inert gases. Often, a vacuum is also pulled on the chamber containing the hot pressing apparatus to allow for the removal of reactive gases that may evolve during heating of the material.
A major disadvantage of this method of consolidation is that the cycle time for hot pressing increases with temperature, making the process slow at very high temperature. It is a further disadvantage that in order to utilize effectively the space available in a hot pressing furnace, it is necessary to assemble a series of parts into a "stack". This procedure can be intricate and not readily adapted to automated production.
Even more disadvantageous from the standpoint of commercial production of high performance structural ceramics is that it is impossible to use simple uniaxial hot pressing to make parts of complex shape. For example, turbine rotors that have intricate curved shapes and substantial variation in the thickness cannot be hot pressed.
To make an article of complex shape, the processing that is generally used has involved "performing" the powder into a partially densified article having the desired shape of the finished part, then further consolidating the pre-form in a second step. Methods commonly used to make the pre-form include die pressing, slip casting, injection molding and extrusion. Pre-forms can then be further consolidated by one or a combination of operations that include cold isostatic pressing, hot pressing, hot isostatic pressing (HIPing), cold and hot forging, hot extrusion and pressureless sintering.
Hot isostatic pressing or HIPing involves sealing a pre-form in an evacuated flexible container and inserting the cladded part into a heating chamber and heating to the sintering temperature. The container can be a thin metal sheath that can be welded, evacuated and sealed. If the metal cladding material is properly selected, it will deform to the shape of the preform and transmit pressure uniformly and isostatically to the part being consolidated. Pressure is applied by compression of the gas in the surrounding chamber. For ceramic components that require very high temperatures to achieve consolidation, for example silicon carbide parts, a metal sheath may be impractical and means to encapsulate the part in a glass composition have been developed.
Hot isostatic pressing does provide a means to consolidate metal and ceramic preforms into dense articles of complex shape, and there is a growing use of HIPing in commercial production of powdered metal parts and ceramic components. However, the major use of HIP has been to densify cast metals and to consolidate powder metal parts. HIP technology can be applied to ceramic materials including the high temperature non-oxide ceramics, such as silicon carbide; but the costs are very high. Temperatures in excess of 2000.degree. C. are needed at pressures of 30,000 psi or higher. As in hot pressing, HIPing entails long cycle times. Because the cost of high pressure equipment and systems are higher than for hot pressing, HIPing is more expensive. There has therefore been increasing interest in other methods that would allow production of ceramic components of complex shape at lower cost.
Yet further, one method receiving extensive study by ceramicists is pressureless sintering. Pressureless sintering involves making preforms of the part from ultra-fine, sinter-active ceramic powder, for example by injection molding or cold isostatic pressing, then heating the pre-form to a temperature approaching the melting point of the material(s) to be sintered, causing a shrinkage and consolidation to the desired final size and shape. Pressureless sintering not only can consolidate complex and intricately shaped parts, but it offers the obvious advantage of adaptation to continuous and easily automated production with inherently lower cost of mass production.
Despite already noted advantages of the pressureless sintering method, technical factors may limit the ability to utilize this process to make certain high performance structural ceramic components from certain powdered ceramic materials. This is particularly true in the case of the high melting temperature non-oxide ceramic materials such as the nitrides, carbides, and borides. For example, to achieve a near theoretical density part by pressureless sintering of silicon carbide powders, it is mandatory to use a silicon carbide powder that is all substantially below 1-2 micron particle size, and meeting close chemical specifications with regard to contained oxygen, carbon and trace elements.
Deviation from these specifications will lead to failure to obtain an essentially pore-free and homogeneous flaw-free structure absolutely necessary to obtain the high strength characteristics required in aircraft or automotive engine turbine rotors. Thus, the prosecution of suitably qualified powders meeting the rigorous demands for pressureless sintering involves intricate and careful control that results in relatively high cost starting raw material powders. Furthermore, even with the most careful control during production of powders, pressureless sintering at high temperature requires a finite time during which the preformed powder compact must remain at the sintering temperature to allow for full shrinkage and densification. Inherent with the sintering process is the tendency for growth of the grains causing departure from the idealized ultra-fine, equi-axed microstructure that offers the highest material strength. Powders that undergo crystal phase transformation at or near the sintering temperature are particularly susceptible to grain growth and a resulting decrease in strength properties. This effect is made more pronounced with increased residence time at the sintering temperature.
It can be noted that very high temperatures are needed to sinter materials that are candidates for high performance engine components. As stated, silicon carbide requires temperatures above 2000.degree. C. to achieve a high degree of densification by pressureless sintering. Although it would be highly desirable from the standpoint of minimizing grain growth to rapidly heat a preform to the sintering temperature and limit the time at temperature to no longer than several minutes, and, possibly to residence times of no more than several seconds, there are a number of practical limitations to doing this by a pressureless sintering process. First, sintering furnaces of conventional design are simply not suited to a quick entry or withdrawal of the material being sintered. More important, however, is the fact that gases are likely to evolve from the preform during heat-up due to a combination of desorption, decomposition and chemical reaction. Too rapid a heat-up rate and associated gas evacuation rate can cause mechanical stresses that would fracture the part. And thirdly, too rapid a heating and/or cooling could also cause fracture due to differential expansion or contraction, referred to as "thermal shock". The limits of temperature raise rate and cooling rate during pressureless sintering are different for different materials and are dictated by the specific material being processed and the size and shape of the part.
However, it should be noted in regard to the limits of temperature rise rate and the speed of the sintering cycle that substantially higher cycle rates are possible when the material is being subjected to pressure, for example as in hot pressing. Pressure minimizes cracking due to gas evolution and, if maintained during the densification cycle, it allows a substantially closer approach to achieving theoretical density than by pressureless sintering.
Thus, the present state of the art can be summarized, as follows: the pressureless sintering method for the manufacture of advanced materials by consolidation from powders, and particularly for making high performance ceramics, represents the most attractive method from the standpoint of high volume-low cost production. However, the mechanical properties obtained in pressureless sintered materials are generally not as good as in materials made by application of pressure during sintering, as is the case for hot pressed and/or HIPed products. But these latter methods are either unsuited for making parts of complex shape, and/or are very slow, and/or are unable to be easily or fully automated; thus, they are very expensive.
It is therefore one material object of the present invention to overcome many of the difficulties of the prior art methods, and more particularly to provide a controlled and controllable means for extremely rapid sintering under pressure to achieve consolidated parts of low porosity and having a flaw-free and ultra-fine grain structure.
It is a further object of the present invention to be able to accomplish these basic objectives even with materials that require temperatures well above 2100.degree. C.
It is still further the purpose of the present invention to accomplish the above objectives within seconds using temperature rise rates in excess of 50.degree. C./sec.
Moreover, it is the purpose of this invention to consolidate preformed materials of complex shape under compaction pressures approaching isostatic compaction conditions. It is still further the purpose of this invention to accomplish these objectives using relatively inexpensive apparatus that is simple in construction and operation and that can be readily adapted to automated, controlled, and continuous high volume production.
It is recognized that some of the above objectives of the present invention have been addressed, in certain asects by previous workers. In that regard, descriptions of processes intended as improvements in manufacture of parts by consolidation of powders have been described in the technical literature and in previously issued patents. Examples are the patents issued to Lichti and Hofstatter (U.S. Pat. Nos. 4,539,175 and 4,640,711). These workers have demonstrated that simple tooling similar to that used in hot pressing can in fact be used to consolidate articles of complex shape by first surrounding the article by a spherically shaped particulate medium which has sufficient resiliency to be deformed under high compaction pressures without excessive breakage and thereby transmit the compaction pressure in a nearly isostatic manner to the part being consolidated. The Lichti-Hofstatter patents (U.S. Pat. Nos. 4,539,175 and 4,640,711) note the advantages provided by using Superior Graphite Co. spherical graphitic carbon product (9400 Series) as the preferred pressure transmitting particulate medium.
Although the consolidation methods of Lichti and Hofstatter theoretically may offer significant advantages in consolidation of pre-formed articles, it has in actual practice been found quite difficult to apply and to control the described procedure for materials requiring very high temperatures for consolidation--for example, for materials that must be heated to above 1500.degree. C. (2732.degree. F.) to achieve densification. U.S. Pat. No. 4,539,175 to Lichti et. al. notes that the consolidation process takes place after the heated preform is placed in a bed of previously heated carbonaceous particles (Col. 4, lines 35 et seq.). It is further stated that although the graphite particles can be heated inductively to 4000.degree. F. (2204.degree. C.), oxidation is significant above 800.degree. F. Hence, it follows that the operation step at temperatures approaching 4000.degree. F. (2204.degree. C.) would become extremely difficult. U.S. Pat. No. 4,539,175 specifically notes that heating to the necessary temperature is done before compaction.
U.S. Pat. No. 4,640,711 describes again the consolidation process thereof and notes the use of a non-graphitic spherical carbon and mixtures of graphite and non-graphitic spherical carbon and mixtures of sperical carbon and/or spherical graphitic carbon and ceramic particles as the pressure transmitting medium. U.S. Pat. No. 4,640,711 is again specific in noting that the preform and the bed of particles are to be at elevated temperature before pressurization. Thus, the patented processes of the Lichti and Hofstatter patents require that both the particulate medium and the pre-form be preheated independently and then individually transported to the compaction apparatus in sequence to allow the pre-form to be immersed within the medium prior to compaction. Such transfer of materials after heating becomes extremely difficult at temperatures needed to consolidate non-oxide ceramics, such as silicon carbide (1990.degree.-2150.degree. C.), and apparatus to accomplish the transfer is complex, expensive and wasteful of energy.
It is further recognized that a process has been reported by Eliezer and co-workers at the University of Texas referred to as High-Energy, High-Rate Consolidation, which utilizes a very high energy electrical discharge, not through a bed, but rather directly to the pre-form while applying compaction pressure to the preform. In this process, electrical energy is transmitted directly through the material being consolidated, and thus can be used only to consolidate those materials having sufficient electrical conductivity. By using a suitably electrically conductive particulate medium, the present invention is not so restricted, and material with no electrical conductivity can be consolidated by the process hereof. Yet further, since the process described by Eliezer et al. is conducted in a manner similar to hot pressing, it is no more suited than hot pressing for making components of complex shape.
In view of the Lichti-Hofstatter and Eliezer et al. processes, it is still another objective of the present invention to utilize the principle of consolidation by compacting a free flowing solid particulate medium as a near isostatic pressure transmitter, but to accomplish heating and compaction essentially simultaneously by a rapid, convenient and energy efficient means. It is a further object of the present invention to include embodiments utilizing a bed material which is not confined to the narrowly specific bed material of the prior art.
It is also an object of the present invention to control the resistivity of the bed material for use in the electroconsolidation process of the present invention according to the following principles.
The important purpose of developing a means to control the specific resistance of the medium relates to optimizing conditions for any particular application hereof. The system hereof in electrical terms in certain embodiments is inherently a relatively low resistance system. That is, for a given power application (i.e., 10 Kw, 100 Kw, etc.) the applied voltage will be low and the current high. The reason for this phenomenon is that it is desired to have the materials of construction, specifically the top and bottom plungers (parts 52 and 16 hereof) which carry the electrical current, made of a very low resistance material (such as, for example, graphite, copper, etc.), to avoid resistive heating of the apparatus and the resulting loss of temperature control and extraneous power losses. Thus, the main electrical resistance of the system will be that of the pressing medium.
One preferred pressing medium is a graphitic carbon, such as Superior Graphite Special Graphitic Carbon 9400 Grade, produced by Superior Graphite Co., Chicago, Ill. This material has the desired excellent flow characteristics and elastic modules (resiliency) and inert character to work as a pseudo-isostatic pressure transmitting medium. However, being a graphitized carbon, its resistivity is relatively low (0.03-0.05 ohm-cm). Low system resistance has the disadvantage of requiring relatively high current to develop the needed electrothermal conversion of electrical energy into heat by means of electrical resistive heating. The apparatus must therefore be designed with the ability to operate with high current flow that requires connecting cables of high current rating, or possibly the use of water cooling. This adds significantly to the cost of the apparatus. It is readily calculated from established electrical system relationships the amount of current that must be passed through the medium to achieve the electrothermal conversion energy release necessary to raise the preform temperature to the sintering temperature within a desired time cycle. For example, to achieve an energy release of 50 Kw with a system offering only 2.0.times.10.sup.-3 ohms resistance, it will be necessary to pass 5,000 amps through the apparatus. By increasing the system resistance to 8.0.times.10.sup.-3 ohms, the required current can be reduced to 2,500 amps with a corresponding reduction made in the current rating of the auxiliary process equipment, bus bars and connecting cables. Therefore, in general, it is desired to have a pressing medium having a relatively high specific resistance.
Actually, the resistivity of a packed bed of a particulate material is not directly measurable as in the case of a solid material that can be cut into a specific shape and is at the full or theoretical density of the particular material. Resistivity of a packed bed of particles is strongly dependent on the applied pressure. Thus, the reported resistivity of a particular medium is defined by the method of measurement and more specifically to the applied pressure and to the size distribution of that material as both conditions affect the degree of packing of the material.
Various methods for measuring the electrical resistivity of particulate carbon materials are known in the prior art. It can also be noted in the prior art that the specific resistance of particulate carbons is very dependent on the pressure applied to bed of the carbon. See "Electrical Properties of Carbons--Resistance of Powder Materials, Carbon, Vol. 24 pp. 337-41 (1986). Thus, to some degree in the electroconsolidation processes hereof the system resistance may be influenced by the pressure being applied. However, to achieve the highest degree of densification, it may be desireable to apply the highest possible pressure, and therefore the use of pressure as a means to increase the system resistance may not in reality constitute a viable method for that purpose. It is therefore, necessary to increase the specific resistance of the particulate medium material, according to the practices and principles of the present invention.
The specific resistivity of packed materials has been determined to be dependent on the degree of particle-to-particle contact, which determines the actual inter-particle area of contact, and thus the resistance of the contact. Thus, the application of pressure on the packed bed serves mainly to increase the area of inter-particle contact by compressing particles against each other. This is especially true of softer more elastic materials such as graphitic carbons.
It is further recognized also that surface hardness is also an important factor in the electrical resistance of particle-to-particle contact in a packed bed of particles, just as it is in the contact resistance between bulk solid materials. For example, the contact resistance between solid graphite and highly conductive solid copper is almost twice as much as between solid graphite and graphite, even though the resistivity of graphite is more than 1,000 times that of copper. This results because graphite is a relatively soft material that is much more readily deformed under pressure, and thus effectuates a more highly effective area of contact. Moreover, the cleanliness of the surface and any film(s) on the surface also may significantly affect contact resistance.
Based upon the forgoing, it is therefore a further aspect of the present invention that the contact resistance, and therefore the resistivity of, a packed bed of graphitic carbon particles can be increased by introducing film-forming materials that tend to coat the surface of the particles. More specifically, as set forth in the present invention, if films can be formed on the surface of the graphitic carbon particles that were harder than graphite, the electrical resistivity of packed beds of this product is thereby rendered substantially greater than that of the initial bed of graphitic carbon particles. Furthermore, materials which react with the graphitic carbon surfaces to form hard compounds stable at high temperature have special advantages for use in the electroconsolidation process of the present invention, particularly for consolidation processes requiring high temperatures. Thus, the class of film forming compounds of the present invention comprises hard, high temperature stable compounds such has carbides, borides, nitrides and related chemical complexes such as carbo-nitrides, etc., the use and composition of which are described in greater detail hereinbelow.